In conventional methods for broadcast distribution of information, messages have been addressed to selected recipients distributed throughout a network by including a distribution list as part of a header which is transmitted with each message.
For example, U.S. Pat. No. 4,138,735, issued Feb. 6, 1979 to Allocca, et al., discloses a method for transmitting data blocks that will be accepted only by specifically addressed receiving stations. Each data block includes postal rate chart information, a receiving station address, a "rate revision number," and a "check sum" (or "check characters") used for verifying accurate reception and processing of the block. Each receiving station will process a block only after it verifies that an address transmitted as part of the block matches a preset address stored in the receiving station.
If the addresses match, the receiving station then checks whether the transmitted rate revision number is greater than a rate revision number stored in the receiving station. If not, the block is not accepted. On the other hand, if the transmitted revision number exceeds the stored revision number, the block is accepted and written into memory. After a block is written into a receiving station's memory, an error computation is performed (using the transmitted check sum or check characters) to determine whether the block has been accurately received and written into memory. If and only if the error computation indicates that the block has been accurately received and written into memory, a rate revision number in the receiving station is incremented.
Each block is cyclically retransmitted. A receiving station will not accept a retransmitted block if it has previously accepted a previous transmission of the same block, successfully performed an error computation on the previously accepted block, and incremented the above-described stored rate revision number upon successful performance of the error computation. If the previous error computation was not successfully performed, then the rate revision number stored within the receiving station will not have been incremented. Thus, if the previous error computation was not successfully performed, upon comparing the rate revision number in the retransmitted block with the unincremented rate revision number stored within the station, the station will accept the retransmited block (and thereafter will perform another error computation, and so on).
However, the conventional method disclosed in U.S. Pat. No. 4,138,735 is inefficient where the transmitted blocks are lengthy (i.e. where each block includes much data). In this case, it is relatively likely that errors in transmission or in processing at the receiving station will occur, so that it is relatively likely that a lengthy message will need to be retransmitted many times before it is finally accurately received and processed.
In the United States, the carrier frequencies of FM radio stations are separated by 200 kHz, in accordance with Federal Communication Commission regulations. Conventional stereophonic FM radio signals that are broadcast over an FM channel typically occupy only a portion of the full 200 kHz band allocated to the channel (i.e., the frequency band within 53 kHz of the carrier frequency). The unused portion of the FM channel occupying the allocated band between 53 kHz and 100 kHz from the carrier frequency is known as the Subsidiary Communications Authorization (SCA) or the SCA band.
Methods and systems have been developed for transmitting FM signals (for example, the commercial-free music sometimes heard in restaurants and stores) in the SCA band of an FM channel. However, an efficient system for transmitting digitally encoded messages in the SCA band of an FM channel using conventional transmission technology must ensure accurate reception of the messages notwithstanding substantial and unpredictably varying noise inherent in broadcast transmission. Since each message retransmission is time-consuming and costly, it is desirable that an efficient system of this type will ensure accurate reception of each message with a minimum number of message retransmissions.
U.S. patent application Ser. No. 134,943, filed Dec. 18, 1987, now abandoned, by David C. Jenkins, which is assigned to the assignee of the present application, is of background relevance to the present application.